Invasive Plants: Species Out of Control

Invasive plants may be alien or native, but all have the ability to become superabundant at the expense of neighboring plants.

The effects of invasive plants on neighboring plants vary, but each has the ability to become established and abundant at the expense of native species. These plants grow around, on top of, or below neighbors and crowd them out – sometimes killing them in the process.

Asiatic bittersweet (Celastrus orbiculatus) and Japanese honeysuckle (Lonicera japonica) grow closely entwined around supporting plants and gradually kill their host plants by shading their leaves, starving them for sunlight, and squeezing their circulatory systems.

Like the tropical strangling fig, these plants are often strong enough that they can support the dead host trunk and hold up the tree even as the trunk rots.

In addition to propagation by seed, Asiatic bittersweet rapidly grows outward by by producing hordes of orange underground rhizomes that spread several yards (meters) from the parent plant. Japanese honeysuckle sends out similar numbers of aboveground runners that root where they touch the ground.

Each rhizome or runner generates buds that grow above ground and search for a new support to climb upon. When pulled up, they fragment and each fragment produces several new shoots. Thus, these species are extremely difficult to eradicate.

Barberry (Berberis vulgaris)is a thorny shrub that grows rapidly and densely. The lack of light and fallen leaves beneath the bush produces a dead zone there.

Barberry has brightly colored (lemon yellow) rhizomes that spread from the parent and produce satellite bushes at some distance from the parent. Thus, a single barberry can, in short time, become a hedgerow or even overgrow a plot of ground in a monoculture.

Digging the plant (after pruning it to the ground) will eliminate the parent and diligent attention to the plant’s surrounding area will eventually remove volunteer sports and rhizomes.

Multiflora rose (Rosa multiflora)is another thorny shrub that grows rapidly and densely. In addition to shading the ground below the plant, this species produces a growth inhibitor in its leaves that prevents the seeds of other species from germinating.

Like the other species listed here, underground rhizomes help the plants to spread rather rapidly. Although each is spaced several feet from the parent, a field will rapidly become impenetrable because of the rapid growth and dense thorns of this species. One (almost) redeeming feature of multiflora rose is that the gray catbird (Dumetella carolinensis) prefers to nest in it over almost any other plant.

Winged burning bush (Euonymus alata) will grow several feet high and wide, but spreads only slowly through underground rhizomes. Once the parent is removed, it quickly sends up volunteers from the roots. These have to be removed over the course of a year or two. The greatest dispersal of this species is by birds.

Pokeweed (Phytolacca americanus) forms a deep vertical tap root with several horizontal branch roots that spread laterally some distance from the parent. Pulling this plant up merely breaks off the tap root and each fragment of root produces a new bud that grows into a new plant. The berries of this native North American plant form a substantial summer and early autumn component of birds’ diets.

Virgin’s bower (Clematis virginiana) is a North American native clematis that produces legions of flowers, each of which can form a dozen or more wind-blown seeds. Though many birds and small mammals eat these, many escape to germinate.

Their roots are composed of a mass of tough fibers spreading from the bottom of the stem. When pulled up, these roots break and, like most other invasives, produce new buds.

The plant completely covers and smothers its host which dies and falls to the ground carrying the bower with it. On the ground, the bower stems roots and dozens to hundreds of new stems begin their search for hosts on which to entwine and reach for sunlight.

Yellow star thistle (Centaurea solstitialis) is an Asiatic plant that invades and replaces the chaparral and grasslands vegetation in California and other western states. Fibrous rootstocks make this weed difficult to pull up.

Numerous seeds are preyed on by small birds and mammals, while spiny leaves and branches. Spines on flower heads, leaves, and branches combine with neurological toxins stored in mature leaves and stems to reduce herbivory by deer and other large mammals. The leaves are toxic to horses.

This species rapidly forms monocultures that cause excessive water transpiration, replace native vegetation, and serve as centers for dispersal to new habitats as seeds are blown by wind and hitch rides on animals, birds, and machinery or the seeds heads are harvested with hay and transported to new sites.

There are many more invasive plants, kudzu, Chinese wisteria, dandelion, purple loosestrife, phragmites, water hyacinth and other introduced aquarium plants, hedging and landscaping plants, knapweeds, grasses, and others.

All pose problems to native plants and (often) animals, and control becomes nearly impossible. Conscientious gardeners attempt to keep their ornamentals under control, but that is not always possible with wind or bird dispersed species.

Often these plants are dispersed by birds that depend on their fruit for nutrition. Thus, eradicating well-entrenched species may harm wildlife if native species are not consequently reestablished.

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